1. Field of the Invention
The present invention relates to a quantum entangled photon pair generator, which is applicable to quantum information and communication systems, such as quantum cryptography and quantum computers. In particular, the present invention relates to a quantum entangled photon pair generator which can selectively output either of polarization entangled photon pairs and time-bin entangled photon pairs.
2. Description of the Background Art
The technology for generating quantum entangled photon pairs is key engineering for implementing quantum information and communication systems, such as quantum cryptography and quantum computers, in which the quantum-mechanical behavior of light, or photons, is utilized. In particular, the quantum key delivery technology utilizing quantum entangled photon pairs can expect an application to ultimate, secure encryption communication, and attracts attention in recent years. In the context, the quantum-mechanical behavior means a behavior in accordance with the superposition principle or the like that several different states can be taken at the same time.
As for the quantum entangled photon pairs to be utilized for quantum key distribution, polarization entangled photon pairs and time-bin entangled photon pairs have predominantly been studied so far. The former is presented by, for example, H. C. Lim, et al., “Stable source of high quality telecom-band polarization-entangled photon pairs based on a single, pulse-pumped, short PPLN waveguide”, Optics Express, vol. 16, No. 17, pp. 12460-12468 (2008). The latter is disclosed by, for example, J. F. Dynes, et al., “Efficient entanglement distribution over 200 kilometers”, Optics Express, vol. 17, No. 14, pp. 11440-11449 (2009).
Polarization entangled photon pairs are photon pairs in which the polarizations of individual photons are not determined but the relationship of the polarizations measured is determined, such as parallel or orthogonal to each other. That is, polarization entangled photon pairs are in a state where a photon pair has its plural polarizations in combination superposed to each other and the polarizations are correlated between photon pairs.
Time-bin entangled photon pairs, considering two time slots to be observed in which photons in pair may possibly exist, are photon pairs in which it is not determined in which time slot individual photons exist but is determined the relationship of measurement results in which two photons definitely exist in one and the same time slot. That is, time-bin entangled photon pairs are in a state where photons in pair are distributed to plural time slots for the photon pair to overlap with each other and the photon pairs are correlated in temporal position therebetween.
As a transmission medium of quantum entangled photon pairs, optical fiber can be used. If optical fiber is used as a transmission medium, it is possible to lengthen a quantum key delivery distance due to the lower transmission loss of the optical fiber.
The polarization entangled photon pair generator has high compatibility with a quantum repeater provided on an optical transmitting path, and it is therefore advantageous to utilize it for the long-distance quantum key delivery system using a quantum repeater. The polarization entangled photon pair generator has high compatibility also with a quantum entangled state of electrons utilizing electronic spin, which is expected to become predominant in the computation by quantum computers. The polarization entangled photon pair generator is therefore advantageous to be utilized as an interface between electrons and photons when connecting a plurality of quantum computers in parallel, for example.
However, optical fibers have statistical birefringent property, so that the polarization state of photons after transmitted may generally be fluctuated. When taking account of polarization entangled photon pairs to be applied to a quantum key delivery system using optical fiber, a polarization control mechanism is required for maintaining the polarization state of photons constant.
Meanwhile, in quantum key delivery systems which use time-bin entangled photon pairs, as an asymmetric Mach-Zehnder interferometer which constitutes a receiver of photons, a polarization-independent device may be selected. In that case, a stable system action can be achieved regardless of fluctuation in the polarization state of quantum entangled photon pairs. This feature of quantum key delivery system utilizing time-bin entangled photon pair generator contributes efficiently to a practical use of a quantum key delivery system using optical fiber.
However, there are the following disadvantages in use of time-bin entangled photon pair generator. First, the generation rate of an encryption key is, in principle, about half as high as the generation rate of polarization entangled photon pairs. Moreover, when it is considered that the transmission distance of an encryption key is lengthened using a quantum repeater in a quantum key delivery system, the advantages of polarization independency which is obtained by using time-bin entangled photon pairs cannot efficiently be enjoyed since existing quantum repeaters have polarization dependency.
As described above, polarization entangled photon pairs and the time-bin entangled photon pairs involve their own advantages and disadvantages. Therefore, quantum key delivery systems, quantum computers and the like utilizing quantum entangled photon pairs would be preferable if they are constituted so that polarization entangled photon pairs and time-bin entangled photon pairs can selectively be used in accordance with utility systems. If a quantum entangled photon pair generator is achieved which can selectively generate either of polarization entangled photon pairs and time-bin entangled photon pairs with a simple manner, it would be convenient for applying photon pairs to quantum key delivery systems, quantum computers or the like.
The inventor of the present application has found that, if a system is realized which can select either of a pair of optical pulses of which the polarization planes are orthogonal to each other (which may hereinafter be referred to as polarization excitation optical pulse pair) and a pair of optical pulses which have the same polarization state and exist in different positions on the time axis (which may hereinafter be referred to as consecutive excitation optical pulse pair) to generate the selected pair of optical pulses by a simple manner, it is then possible to selectively generate polarization entangled photon pairs and time-bin entangled photon pairs. In other words, if the polarization excitation optical pulse pair or the consecutive excitation optical pulse pair (which may hereinafter be simply referred to as excitation optical pulse) is utilized as a seedlight pulse for photon pair generation, then the correlated photon pairs including signal photons and idler photons can be generated through a parametric fluorescence process. Then, the correlated photon pairs, which are generated using polarization excitation optical pulse pairs and consecutive excitation optical pulse pairs as a seedlight pulse, are polarization entangled photon pairs and time-bin entangled photon pairs, respectively. Hereinafter, signal photons and idler photons are referred to as a signal light and an idler light, respectively.